Long-term snowpack manipulation promotes large loss of bioavailable nitrogen and phosphorus in a subalpine grassland

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Nutrient retention in ecosystems requires synchrony between the supply of bioavailable nutrients released via mineralization and nutrient uptake by plants. Though disturbance and chronic nutrient loading are known to alter nitrogen (N) and phosphorus (P) dynamics and induce nutrient export, whether long-term shifts in climate affect source-sink synchrony, and ultimately primary productivity, remains uncertain. This is particularly true for snow-dominated ecosystems, which are naturally subject to lags between nutrient inputs and uptake. To address how climate change may affect nutrient source-sink synchrony we examined the impacts of deepened snowpack on N and P losses in a subalpine grassland in the Northern Rocky Mountains, USA, where we have experimentally increased snowpack depths by two- and four-times ambient snow for 45 years. Long-term snow addition resulted in remarkably high levels of bioavailable-N leaching (up to 16 kg ha^-1 year^-1) that were 11-80 times higher than those under ambient snowpack. Estimated bioavailable-P losses also increased with snow addition, but to a lesser degree (up to 0.3 kg ha^-1 year^-1), indicating greater enhancement of N losses over P losses during snowmelt. Because these losses could not be explained by changes in nutrient inputs in snowpack or by changes in plant-soil turnover, our results suggest that high bioavailable-N leaching under deep snowpack originates not from a lack of N limitation of plant productivity, but rather from enhanced subnivean microbial processes followed by snowmelt leaching prior to the growing season. This is supported by reduced soil N pools in the snow treatments. Snow-dominated regions are projected to experience shifts in seasonal snowpack regime. These shifts may ultimately affect the stoichiometric balance between available N and P and future plant productivity.